This finding has significant implications for the study of exoplanetary atmospheres and the search for water-rich planets throughout the galaxy.

GJ 9827d, which is no larger than twice Earth's diameter, could serve as an example of potential water-rich worlds found elsewhere in our galaxy. However, the planet's scorching temperature of 800 degrees Fahrenheit renders it uninhabitable, resembling a steamy world akin to Venus.

Team member Björn Benneke from the Trottier Institute for Research on Exoplanets at the Université de Montréal expressed the importance of this discovery, stating, "This would be the first time that we can directly show through an atmospheric detection that these planets with water-rich atmospheres can actually exist around other stars." Laura Kreidberg from the Max Planck Institute for Astronomy in Heidelberg, Germany, a co-principal investigator, added that finding water on a planet of this size is a significant milestone, bringing scientists closer to characterizing truly Earth-like worlds.

However, scientists are still uncertain about the nature of GJ 9827d's atmosphere. It remains unclear whether the detected water vapor indicates a hydrogen-rich atmosphere with small amounts of water or an atmosphere predominantly composed of water left after a primeval hydrogen/helium atmosphere evaporated due to stellar radiation.

The observing program, led by principal investigator Ian Crossfield of Kansas University, was specifically designed to detect water vapor in GJ 9827d's atmosphere. Pierre-Alexis Roy, the lead author of the science paper from the Trottier Institute for Research on Exoplanets, explained that either scenario, whether water vapor dominates a hydrogen-rich atmosphere or exists in small quantities, would provide exciting insights into the prevalence and diversity of rocky exoplanet atmospheres.

Although GJ 9827d's extreme temperature makes it inhospitable, the presence of a residual water-rich atmosphere suggests that the planet may have formed farther away from its host star, where water was available in the form of ice. It is speculated that the planet later migrated closer to the star, receiving more radiation that caused its hydrogen-rich atmosphere to heat and escape due to its weak gravity.

The Hubble Space Telescope's observations of GJ 9827d were conducted during 11 transits over a three-year period. During these transits, starlight passing through the planet's atmosphere revealed the spectral fingerprint of water molecules. Hubble's ability to probe water vapor above clouds, if they exist, provides valuable insights into the planet's atmosphere.

Thomas Greene, an astrophysicist at NASA's Ames Research Center, noted that observing water on exoplanets serves as a gateway to discovering other elements and molecules, paving the way for future studies using the upcoming James Webb Space Telescope (JWST). The JWST's infrared observations will provide a more comprehensive inventory of a planet's elements, allowing scientists to compare them with their host stars and gain insights into the planet's formation.

GJ 9827d was initially discovered in 2017 by NASA's Kepler Space Telescope. It orbits a red dwarf star in the Pisces constellation and completes an orbit every 6.2 days. This latest discovery adds to our understanding of the composition and evolution of exoplanetary atmospheres, hinting at the possibility of finding water-rich planets throughout our galaxy.

The Hubble Space Telescope, a joint project between NASA and ESA, is managed by NASA's Goddard Space Flight Center and operated for NASA by the Association of Universities for Research in Astronomy through the Space Telescope Science Institute in Baltimore, Maryland.